The most successful approaches to the treatment of viral diseases are based on the inhibition of viral DNA replication. The inhibitors are nucleoside analogs which, when incorporated into the DNA of the virus, prevent further polymerization. The nucleosides are actually prodrugs of the active 5'-triphosphate metabolites produced via a sequence of three phosphorylation steps catalyzed by cellular or virally encoded enzymes. The nucleosides are administered in the non-phosphorylated state to promote transit into human cells. However, not all nucleoside analogs are phosphorylated with the same efficiency, and some, whose triphosphates are potent inhibitors of viral DNA polymerase are poor antiviral agents when administered as the nucleoside. This may be due to the specificity of the kinases, or because of insufficient amounts of these enzymes. If the nucleosides could be delivered intracellularly as their monophosphates, thus bypassing the first critical phosphorylation step, their performance as antivirals would be substantially improved. This proposal describes the synthesis and evaluation of a unique class of activated nucleoside monophosphate triesters in which the charged phosphate moeity is masked with ketol groups. These previously unknown ketol nucleoside monophosphate triesters are expected to be lipophilic enough for cell membrane penetration and reactive enough for hydrolytic unmasking within the cell. In this way, the nucleoside monophosphates can be liberated intracellularly where they are expected to exhibit enhanced conversion to the triphosphate. Ketol phosphate triesters are exceptionally reactive toward hydrolysis. Thus the intracellular hydrolytic removal of the first ketol group from the masked nucleotides is expected to proceed non-enzymatically and regiospecifically to afford nucleoside monophosphate diesters. Hydrolysis of the resulting diester is likely to be partitioned between enzymatic and nonenzymatic hydrolysis pathways. This strategy calls for the chemical packaging of proven antivirals in a new more active prodrug form. If this approach is successful, it will lead to an increase in the number and variety of clinical treatments for viral infections.